因為需要把線上的apk檔案下載到本地，一上來就使用了FileWriter的方式進行檔案寫入，結果發現下載到本地的apk會提示安裝包損壞，經過排查，原來FileWriter是使用的字元寫入的方式，而可執行的apk實際是二進位制檔案，這樣在位元組讀取，字元寫入的時候檔案顯然也發生了變化，明顯的差異就是檔案大小變成了兩倍……

官方解釋是：

<span style="white-space:pre">	</span>FileOutputStream is meant for writing streams of raw bytes such as image data. For writing streams of characters, consider using FileWriter.
 

FileWriter程式碼示例：

	public String GetMergedMap(String buildurl, String typeregx,
		String contentregx, String localfile) {
		ArrayList<String> urls = new ArrayList<String>();
		urls = getLinkURL(buildurl, typeregx);

		File local = new File(localfile);
		if (local.exists()) {
			local.delete();
		}
		try {
			local.createNewFile();
			FileWriter fw = new FileWriter(localfile);
			OkHttpClient httpclient = new OkHttpClient();
			Request request;

			for (int i = 0; i < urls.size(); i++) {
				String url = urls.get(i);
				logger.info("{}", "下載：" + url);
				request = new Request.Builder().url(url).build();
				Response response = httpclient.newCall(request).execute();
				if (response.isSuccessful()) {
					InputStream is = response.body().byteStream();
					BufferedReader br = new BufferedReader(
							new InputStreamReader(is));
					String line;
					while ((line = br.readLine()) != null) {
						if (line.matches(contentregx)) {
							fw.append(line + "\r\n");
							fw.flush();
						}
					}
				} else {
					logger.error("{}", "請求失敗 " + response.headers().get("code"));
					System.exit(0);
				}

			}
			fw.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			logger.error("{}", "map檔案下載失敗");
		}
		finally {
			logger.error("{}", "全部下載完畢");
			return localfile;
		}
	}
 

<span style="white-space:pre">	</span>public String downloadApkOnline(String url, String localfile) {
		File local = new File(localfile);
		if (local.exists()) {
			local.delete();
		}
		try {
			local.createNewFile();
			int chByte = 0;
			OkHttpClient httpclient = new OkHttpClient();
			Request request = new Request.Builder().url(url).build();
			Response response = httpclient.newCall(request).execute();

			InputStream is = response.body().byteStream();
			chByte =  is.read();

			FileOutputStream fos = new FileOutputStream(localfile);
			while(chByte != -1)
			{
				fos.write(chByte);
				chByte =  is.read();
			}
			fos.flush();
			fos.close();
			is.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			logger.error("{}", "下載失敗");
		} finally {
			return localfile;
		}
	}
 

<span style="white-space:pre">	</span>public String  downloadApkByBuffer(String url, String localfile)
	{
		File local = new File(localfile);
		if (local.exists()) {
			local.delete();
		}
		try {
			local.createNewFile();
			int chByte = 0;
			OkHttpClient httpclient = new OkHttpClient();
			Request request = new Request.Builder().url(url).build();
			Response response = httpclient.newCall(request).execute();

			InputStream is = response.body().byteStream();
			chByte =  is.read();

			FileOutputStream fos = new FileOutputStream(localfile);
			BufferedOutputStream bos = new BufferedOutputStream(fos);
			while(chByte != -1)
			{
				bos.write(chByte);
				chByte =  is.read();
			}
			bos.flush();
			bos.close();
			fos.close();
			is.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			logger.error("{}", "下載失敗");
		} finally {
			return localfile;
		}
	}